Linear compressor

ABSTRACT

A linear compressor having a compressor chamber with an end wall that includes a recess; a piston that moves in an oscillating manner in the compressor chamber; and a proximity sensor that is disposed on the end wall of the compressor chamber to detect an approach of the piston to the end wall. The proximity sensor is adjacent to the recess in the end wall and the piston has a projection which engages in the recess at a top dead center of the piston movement.

The present invention relates to a linear compressor, in particular for use as a refrigerant compressor in a refrigerator, in particular a household refrigerator. Such a linear compressor is known for example from DE 10 2004 010 403 A1.

The drive of such a linear compressor conventionally comprises at least one excitation winding to which current can be applied to generate a magnetic alternating field and a magnetic armature that can be moved between two reversal points in the alternating field of this excitation winding. Such linear drives are of particular interest as drives for compressors, since they can drive the reversing movement of the piston of such a compressor directly in a simple structure, while the use of a rotary drive requires a mechanical system which comprises for example a crankshaft and a piston rod connected thereto, to convert the rotational movement of the drive to the desired reversing movement of the piston. This mechanical system incurs manufacturing costs and results in friction losses.

However while a structure with a crankshaft and piston rod predefines the movement amplitude of a compressor piston exactly, the movement amplitude of the piston of a linear compressor is generally not fixed but varies as a function of the electrical power to which the excitation coil is subjected and the pressures at the inlet and outlet of the compressor. A small piston amplitude means that at the top dead center of the piston movement the dead volume is large and the generated overpressure is small. To achieve a high level of compressor efficiency, the dead volume must be kept as small as possible, in other words at its top dead center the piston must come as near as possible to a end wall of the compressor chamber. At the same time however it also must not strike the end wall, as this would result in rapid wear or destruction of the compressor.

It is therefore necessary to monitor the amplitude of the linear drive during operation and to regulate it so that the dead volume of the compressor is as small as possible but at the same time the piston is reliably prevented from striking the end wall of the compressor chamber.

Various concept have been discussed with a view to achieving this goal, such as for example the optical scanning of markings applied to the armature of the linear compressor, mechanical contacts, which are actuated by the piston as it approaches the end wall or an inductive sensor, which is attached to the low-pressure side of the end wall, to detect the approach of the piston through this.

The measurement accuracy of the inductive proximity sensor is generally inversely proportional to the distance between the sensor and the object to be detected. This produces the dilemma that although it is desirable to detect the approach of the piston to the end wall from a long distance, in order to be able to prevent a collision by a minor correction of the excitation current, because of the uncertainty of distance detection however it remains uncertain whether a correction of the excitation current that appears necessary during detection from a long distance is actually necessary or the presumed need for correction only results from a measurement inaccuracy, and/or whether the correction is anyway sufficient to prevent the end wall being struck. The shorter the distance, at which the sensor detects the piston, the more significant the corrections that are required to the excitation current to prevent a collision. It is therefore desirable to create a linear compressor, in which these basic problems have little impact.

This object is achieved in that with a linear compressor with a piston that can be moved in an oscillating manner in a compressor chamber and a proximity sensor disposed on a end wall of the compressor chamber to detect an approach of the piston to the end wall, the proximity sensor is adjacent to a recess in the end wall and the piston has a projection which engages in the recess at the top dead center of the piston movement. The engaging contours of the recess and projection mean that the axial longitudinal distance between the end face of the piston projecting forward in the direction of the end wall of the compressor chamber and the positional plane of the inside of the end wall of the compressor chamber does not correspond to the residual freedom of movement of the piston, in other words the path the piston can still cover in the direction of the end wall, before it collides with it. Rather the residual freedom of movement of the piston is longer by roughly the extent of the projection in relation to the end face of the piston enclosing it. This produces a defined safety zone for the forward travel of the piston that is still possible in the direction of the end wall, before the end face of the piston, which is disposed round the outside of the projection and offset backward in relation to this by its axial length, comes into contact with the end face of the end wall which is inside the chamber and disposed around the recess. The axial distance between the sensor and the projection of the piston is thus measured or detected as practically zero, as soon as the projection engages in the recess, without the residual freedom of movement of the piston disappearing at the same time. As soon as the sensor detects that the axial longitudinal distance between the projection and the end wall surface around the recess is equal to zero, the piston is only allowed a defined forward travel path, which is smaller than the axial length of the projection, to avoid an unwanted collision between the piston end face disposed around the outside of the projection, preferably concentrically, and the end face of the end wall of the compressor chamber inside the chamber, which is disposed on the outside, preferably concentrically. This prior identification or prior detection of the actual position of the front piston end face, before it comes into contact with the end wall of the compressor chamber, allows specific and defined control of the further piston travel movement in the direction of the front end wall of the compressor chamber. This improves the ratio of measurement accuracy to residual freedom of movement and allows accurate control of the piston movement with few control interventions.

The end wall of the compressor chamber is in particular formed by its valve plate.

The proximity sensor is preferably a low-cost inductive sensor.

In order to minimize an air gap between the projection and the recess quickly as insertion starts, the recess and projection preferably feature side faces that are parallel to the axial movement direction. At the start of insertion there is therefore an abrupt rise in inductivity that can be detected easily and precisely.

In order to minimize the distance between the sensor and piston further, the proximity sensor is preferably disposed on the side of the end wall facing the compressor chamber. As the proximity sensor does not therefore have to detect the piston through the end wall, the end wall can be made of metal. As well as being highly durable, this has the advantage of protecting the sensor from external magnetic fields, which could impair its measurement accuracy.

One coil of the sensor is preferably disposed around the recess.

Because the recess is located on the longitudinal axis of the cylindrical compressor chamber, it can be ensured that the projection always enters the recess regardless of any rotation of the piston about its longitudinal axis. There is therefore no need to limit the freedom of rotational movement of the piston, which in turn simplifies the structure of the compressor.

The recess can at the same time represent a passage for fluid circulated by the compressor. The passage is preferably an outlet opening of the compressor chamber, as this does not require a valve body on the side of the compressor chamber, the movement of which could falsify the sensor detection results.

In order to prevent falsification of the detection results by the movement of the valve body, the latter is preferably made of a dielectric material, which is practically “invisible” to the magnetic sensor.

Other developments of the invention are set out in the subclaims.

The invention and its developments together with their advantages are described in more detail below based on drawings, in which:

FIG. 1 shows a schematic longitudinal section of an advantageous exemplary embodiment of an inventive linear compressor; and

FIG. 2 shows a perspective view of the end wall of the compressor chamber of such a linear compressor.

The drive unit of the linear compressor shown in FIG. 1 for a household refrigerator 1 in the known manner comprises an E-shaped metal yoke 1 with three parallel fingers, around the middle finger of which an excitation winding 2 is disposed. A magnetic armature 4 is suspended from leaf springs 5 so that it can be moved in an oscillating manner in an air gap between the yoke 1 and an opposite yoke 3. The excitation winding 2 can be subjected by a control circuit (not shown) to an alternating current, which generates magnetic fields of temporally alternating orientation in the air gap, these driving the movement of the armature 4. The yoke 3 may be passive, as shown in FIG. 1, or it may be disposed as a mirror image of the yoke 1 and likewise fitted with an excitation winding.

The armature 4 drives a cylindrical piston 7 in a compressor chamber 8 by way of a piston rod 6. The compressor chamber 8 is closed off at an end facing away from the drive unit by an end wall 9, in which openings 12, 13 fitted with valves 10, 11 are formed. The end wall is therefore preferably configured as a valve plate of the compressor chamber. The openings 12, 13 form intake and respectively discharge openings for a refrigerant circulated by the compressor. One of the openings, the discharge opening 13, is located precisely on the longitudinal axis of the compressor chamber 8. A cylindrical projection 14 on the piston 7 is dimensioned and positioned to enter the opening 13 with little clearance, shortly before the piston 7 touches the end wall 9.

As shown in particular in FIG. 2, a coil 15 of an inductive sensor extends around the opening 13. The inductivity of the coil 15 is a function of the magnetic permeability of its surroundings and therefore of the distance between the piston 7 and the end wall 9. As long as the magnetic field—shown with broken lines in FIG. 1—of the coil 15 essentially does not reach the piston 7, the inductivity of the coil 15 and therefore the measurement signal of the inductive sensor, of which it is part, is largely independent of the position of the piston 7. When the piston 7, on its path to the top dead center, starts to enter the field, the inductivity gradually increases and a significant growth in inductivity is noted when the projection 14 approaches the opening 13 and starts to enter it. The width of an air gap between the metal end wall 9 and the likewise metal piston 7 is reduced almost to zero here in the region of the projection 14, even though before contact with the end wall 9 the piston 7 still has a residual freedom of movement, which corresponds roughly to the axial length of the projection 14. The inductive sensor is therefore able to supply a measurement signal indicating a drop below a predefined distance between the piston 7 and the end wall 9 with much greater accuracy than with a conventional piston with a flat end face.

Since the coil is located on the side of the end wall 9 facing the compressor chamber 8, the detection result of the inductive sensor is independent of the thickness of the end wall 9. In practice it is possible expediently to select the thickness of the end wall to correspond roughly to a distance between the piston and end wall, the dropping below which is to be detected with the aid of the sensor, and the sensor is designed to supply a detection signal in each instance when the projection 14 enters the opening 13, in other words where the inductivity varies the most with piston position and therefore the most precise detection is possible.

One desirable side effect of the projection 14 is a reduction in the dead volume of the compressor chamber 8 at the top reversal point, since the compressed refrigerant is largely driven out of the opening 13.

FIG. 2 shows a detailed perspective view of the end wall 9 and the valves 10, 11 of the compressor from FIG. 1. The projected outline of the piston 7 is shown as a dashed circle 16 on the end wall 9. Drilled holes 17 are formed outside the circle 16, at the four corners of the end wall, to hold screws (not shown), which connect the compressor chamber 8 to a head piece 18 (see FIG. 1) on the other side of the end wall 9 and hold the compressor chamber 8 and the end wall 9 pressed tightly against one another. A further drilled hole 19 connects a pressure-side chamber 20 of the head piece 18 to an annular hollow space 21 enclosing the compressor chamber 8, from which a small quantity of the compressed refrigerant flows through a fine drilled hole back into the compressor chamber 8 to form a pressurized gas bearing for the piston 7.

Two further drilled holes 22 in the end wall 9 serve to attach the valves 10, 11, the structure of which will be examined in more detail below.

Visible within the circle 16 is the central outlet opening 13, the coil 15 inserted into a groove in the end wall 9 concentric to the outlet opening 13, connecting lines 23 of the coil 15, which are passed through tightly packed drilled holes 24 on the side of the end wall 9 facing away from the compressor chamber 8, and the inlet opening 12 in the form of an arc concentric to the outlet opening 13.

The valves 10, 11 are embodied as leaf springs, which are attached to the end wall with the aid of rivets 25 extending through the drilled holes 22. The leaf springs of the valves 11, 12 each have an extended foot 26, in which holes 27 for the rivets 25 are formed, which complement the drilled holes 22, and an elastic tongue 28 projecting from the foot toward the longitudinal axis.

An opening 29 is cut into the tongue 28 of the valve 10 and exposes the outlet opening 13 when the valve is riveted to the end wall 9. An end segment 30 of the tongue 28, which extends in an arc around the opening 29, forms a blocking unit for the inlet valve 10, which covers the inlet opening 12 in the untensioned state.

The tongue 28 of the outlet valve 11 rests against the outside of the drilled outlet hole 13, its tip functioning as the blocking body of the outlet valve 11.

The leaf springs of the valves 10, 11 can be made of an elastic dielectric material, such as a plastic for example, so that the position of the inlet valve 10 does not influence the inductivity of the coil 15 significantly. Alternatively the valve 10 can also be manufactured from spring steel, if its opening 28 is large enough to expose the coil 15.

A coil printed on a printed circuit board can also be used instead of a wire-wound coil 15. 

1-11. (canceled)
 12. A linear compressor, comprising: a compressor chamber having an end wall, the end wall having a recess; a piston that moves in an oscillating manner in the compressor chamber; and a proximity sensor disposed on the end wall of the compressor chamber to detect an approach of the piston to the end wall; wherein the proximity sensor is adjacent to the recess in the end wall; and wherein the piston has a projection which engages in the recess at a top dead center of the piston movement.
 13. The linear compressor of claim 12, wherein the proximity sensor is an inductive sensor.
 14. The linear compressor of claim 12, wherein the recess and the projection have a respective side face that is parallel to an axial movement direction of the piston.
 15. The linear compressor of claim 12, wherein the proximity sensor is disposed on a side of the end wall that faces the compressor chamber.
 16. The linear compressor of claim 15, wherein the end wall is made of metal.
 17. The linear compressor of claim 12, wherein the proximity sensor comprises a coil that encloses the recess.
 18. The linear compressor of claim 12, wherein the compressor is cylindrical and has a longitudinal axis, and wherein the recess is located on the longitudinal axis of the cylindrical compressor chamber.
 19. The linear compressor of claim 12, wherein the recess is a passage for fluid circulated by the linear compressor.
 20. The linear compressor of claim 19, wherein the passage is an outlet opening of the compressor chamber.
 21. The linear compressor of claim 19, wherein a valve body made of dielectric material is assigned to the passage.
 22. A refrigerator, comprising: a linear compressor, the linear compressor having: a compressor chamber having an end wall, the end wall having a recess; a piston that moves in an oscillating manner in the compressor chamber; and a proximity sensor disposed on the end wall of the compressor chamber to detect an approach of the piston to the end wall; wherein the proximity sensor is adjacent to the recess in the end wall; and wherein the piston has a projection which engages in the recess at a top dead center of the piston movement.
 23. The refrigerator of claim 22, wherein the refrigerator is a household refrigerator. 